The present invention relates to the art of injection molding by means of hot runners wherein molten plastic material is fed from a pressure chamber to an injection nozzle and to an injection or mold gate of a die cavity. The molten plastic traverses a sprue channel in a heated structure designed to maintain the unused portion of plastic in its fluid state for injection in the next cycle. The present invention relates specifically to substantially insulation adjacent the injection nozzle resulting in a substantially improved injection molding device and procedure.
In order to maintain the plastic in a molten state the distributor assembly termed the hot runner and the nozzle assembly must be heated. Moreover, in order to function properly, heat expansion of the distributor and nozzle assembly must be accommodated and, while doing so, leakage between the distributor channel system and the nozzles as well as between the nozzles and the mold gate must be prevented. In addition to the foregoing, in designing a hot runner system, it is one of the most important requirements to minimize heat transfer from the nozzles to the gates. As is well known, the gate, i.e. the entry orifice for the plastic into the mold cavity, is usually placed within a separate mold component termed the gate pad. In order to operate at a rapid cycle, the gate pad must be strongly cooled so as to freeze the gate in the least amount of time and thereby to permit the mold to be opened and the finished article to be removed. At the same time, the plastic within the nozzle must remain at molding temperature, because cooling it would reduce the ability of the resin to flow rapidly into the mold cavity and, in the worst case, form a plug within the nozzle which would require cleaning while interrupting the molding operation. In order to minimize heat transfer from nozzle to gate pad, attempts have been made, as will be discussed below, to provide some insulation between the two and/or to minimize contact surfaces between the nozzle tip and the bottom portion of the gate pad. In accordance with the present invention substantially improved insulation between nozzle and gate pad is provided and a substantially improved injection molding device and procedure is obtained.
U.S. Pat. No. 3,741,704 granted June 26, 1973 to Beasley shows a pressed asbestos insulator to reduce thermal losses between the nozzle and the mold cavity. However, alignment of the nozzles and compensation for heat expansion of the nozzle assembly are not provided. The asbestos ring is incompressible for all practical purposes and therefore unable to provide a proper seal over the necessary range of dimensional changes. In addition, of course, the harmful nature of asbestos is well known.
U.S. Pat. Nos. 3,849,048 granted Nov. 19, 1974 to Bielfeldt et al. and 4,212,625 granted July 15, 1980 to Shutt and 4,268,240 granted May 19, 1981 to Rees et al. show the use of the molding resin itself to form an insulating layer between the nozzle and the gate and gate pad, accounting also for alignment of the components and accommodation of the heat expansion. However, if a heat sensitive resin is thus used to form the insulating layer, as shown in these patents, it will tend to degrade in prolonged use, adversely affecting the molded articles. An arrangement according to these patents fails to allow the heat sensitive resin to be continuously flushed out with none remaining to degrade and to contaminate subsequently molded articles.
U.S. Pat. No. 4,043,740 granted Aug. 23, 1977 to Gellert shows a nozzle assembly using a titanium seal to space the nozzle from the cavity. While more suitable for heat sensitive resins than the above, heat exchange between the nozzle and the gate pad is excessive because titanium is a relatively good heat conductor.
U.S. Pat. No. 4,416,608 granted Nov. 22, 1983 to Deardurff shows another method of reducing contact and thereby heat transfer between the nozzle and the gate pad, namely an abrasive surface treatment using, e.g. vapor honing to reduce the contact area between the two components thus reducing heat flow; however, this method is only partially effective at best.
It is therefore a principal objective of the present invention to provide an improved injection molding device and process and especially characterized by improved insulation between the nozzles and the mold gate.
It is a still further object of the present invention to provide a device and process as aforesaid which is readily able to accommodate dimensional changes.
It is an additional object of the present invention to provide a device and process as aforesaid wherein the improved insulation does not degrade with time.
Further objects and advantages of the present invention will appear hereinafter.